This invention relates to a process for manufacturing a container of thermoplastic materials, having a triple-wall structure wherein one laminate is composed of a thermoplastic material differing in chemical and physical properties from the other laminates.
Polyethylene, i.e., one of the typical polyolefin resins, has been widely used for the production of shaped articles such as blow-molded containers because polyethylene is excellent with regard to chemical resistance, impact resistance and flexibility at low temperature, and moisture resistance. Polyethylene is, however, poor with regard to resistance to the transmission therethrough of chemicals such as halogenated hydrocarbons, alcohols, ethers and ketones. For example, the transmission rate of gasoline through polyethylene is scores of times that through nylon-6. This results in the disadvantage that the thickness of the polyethylene vessel used for storing for example gasoline must be increased to a great degree. Further, polyethylene is inferior in resistance to the transmission therethrough of gases other than steam, such as oxygen, carbon dioxide and nitrogen and, accordingly, it cannot be used practically as a container for storing oxidizable, perishable or discolorable substances.
Polyamide resins also have a serious defect in that they exhibit a high transmission rate of water vapour therethrough. However, they do not have the polyethylene defects mentioned above in that they do not exhibit a high transmission rate of such substances as the above-listed chemicals, gases and gasoline.
Thus, if a combination of both the polymers is used as a raw material for a shaped article, it can be naturally expected that plastic shaped articles having characteristics satisfactory from the point of view of practical use are obtainable, which characteristics are not possessed by any plastic articles shaped from the single polymer component. In order to realize this expectation, some proposals have heretofore been made which include, for example, shaping a mixture of polyethylene and polyamide; melt-extruding the two polymers through one annular orifice to form a composite tube composed of at least two tubular laminates, as disclosed in U.S. Pat. No. 3,457,337, and; melt-extruding polyethylene and polyamide through concentrically positioned outer and inner annular orifices and then bringing the extruded two tubes into intimate surface engagement by introducing air inside the inner tube, as disclosed in U.S. Pat. No. 3.082,484. Of these proposals, the procedure of producing a composite tube of the two polymer tubular laminates is advantageous from the point of view that the container has the expected good characteristics resulting from the combination of polyamide and polyethylene. However, the combination of polyamide and polyethylene has the defect that a strong bonding strength is not obtainable and this is unsatisfactory from the point of view of practical use because a strong bonding strength is required particularly at the pinched-off area during the blow-molding operation. This poor bonding strength is due to the fact that polyolefin is non-polar and poor in compatibility with polyamide.
In order to provide a laminated article comprised of polyolefin and other thermoplastic polymers, having a strong bonding strength, it has been proposed to apply a binder comprised of modified olefin polymers such as a copolymer of a predominant amount of ethylene and a minor amount of a modifier such as vinyl acetate, and unsaturated carboxylic acids such as acrylic acid and methacrylic acid and their derivatives, or to employ such modified olefin polymers as the polyolefin component to be laminated with the other thermoplastic polymer. However, these proposals are also unsatisfactory in the points that the lamination procedure is complex, or costly because the modified olefin polymers are expensive, and the resulting bonding strength is not sufficiently improved.
Further, it has been proposed to melt-extrude a composite tube comprised of two tubular laminates, one being polyamide and the other being a mixture of polyolefin and a modified ethylene polymer such as an ethylene-zinc methacrylate copolymer, while the two independent streams of the polymer melts are joined together within a die, followed by the blow-molding, as disclosed in U.S. Pat. No. 3,819,792.
However, such container having a double-wall structure still has the following defects. In the case where the inner laminate is polyamide and the outer laminate is the polyolefin mixture, the containers is not satisfactory suitable for storing foods from a sanitary point of view because: firstly, polyamide usually contains a minor quantity of the low molecular weight component which tends to migrate into the foods, and; secondly, polyamide is liable to be penetrated by water contained in the foods and, hence, the bonding strength between the polyamide inner layer and the polyolefin mixture outer layer gradually decreases over a lengthy period of use, although the container is excellent with regard to resistance to the transmission therethrough of gasoline. In contrast, in the case where the outer laminate is polyamide and the inner is the polyolefin mixture, the outer laminate of the container is liable to absorb water in the air and, hence, gas transmission through the container decreases to a degree, for example, such that transmission rates of gases such as oxygen, carbon dioxide and nitrogen in air of an equilibrium moisture content are approximately twice those in the absolutely dried air. Further, when the composite tube of the two polymer laminates, i.e., polyamide and the polyolefin mixture, is extruded through a die, melt of the polyamide tends to stick to the contacting surface of the outlet of the die and damages the appearance of the parison and hence, the product. This is especially noticeable either when the balance between the flow rates of the two polymer melts is lost or when the inner pressure in the die excessively increases.